When the seals and/or the guiding elements of the piston have to be replaced, it is necessary to detach the guiding bush with respect to the housing, and to pull the piston rod with the piston being connected thereto out off the housing. When guiding elements, seals and/or stripping elements being associated with the piston rod and usually being located in the guiding bush have to be replaced, it is necessary to detach the piston from the piston rod to be capable of accessing the inner opening being located in the guiding bush.
The detachable arrangement and connection of the piston at the piston rod is an important issue for piston and cylinder units. The detachable connection has to be chosen such that the piston can withstand the axial forces produced by the fluid acting upon the piston, and such that it can transmit these forces onto the piston rod.
Furthermore, is it necessary to seal the fluid in a region between the piston and the piston rod. In the case of a threaded connection existing between the piston and the piston rod, it is necessary to prevent unintentional loosening of the connection under operational loads by the piston rotating with respect to the piston rod.
In the art, one differentiates between so called “pre-tensioned connections” and “not-pre-tensioned” connections. In the case of pre-tensioned connections, a tightening moment or torque is applied by the threaded connection such that the piston and the piston rod in the mounted position are pressed against one another in a pre-tensioned way. Usually, it is necessary to harden the front surfaces of the piston and of the piston rod to fulfill the requirements of the acceptable surface pressure, and to realize the required great tightening moments. Such a great tightening moment has substantial advantages during operation of the piston and cylinder unit since the piston is only subjected to a pulsating stress, and the axial force resulting from the tightening moment always remains more than the axial force caused by the fluid. The tightening moment is the first way of securing the piston against unintentional detachment under operational conditions.
It is also known to use a second way of securing the piston. When replacing the seal and/or the guiding element being associated with the piston rod, unintentional loosening of a piston being mounted to a piston rod with a tightening moment is a substantial problem. This is especially true when one realizes that it is often the case that such piston and cylinder units are part of construction machines, excavators and the like which are usually repaired outside and without factory equipment.
Another piston and cylinder generally known in the art includes a piston being pushed upon a protrusion of the piston rod. The opening of the piston is designed to be continuous in an axial direction, but it does not include a thread. The protrusion of the piston rod includes an outer thread on which a locking nut is arranged by screwing after having pushed the piston onto the protrusion. The locking nut has an opening and an associated inner thread. For example, the locking nut may include a plastic material. The plastic material serves as a securing element against unintentional loosening. In this way, a non-positive securing effect against rotation is used.
It is also generally known in the art to combine the piston and the locking nut to attain one common element such that the associated inner thread is located in an opening of the piston. As the securing element, a pin screw being located in the piston in a radial direction is used, the pin screw in the mounted position pressing upon the protrusion of the piston rod. The piston has to include a respective engagement surface for a tool to be capable of applying the necessary great tightening moment during assembly and the respective loosening moment during detachment it is often necessary to use special tools.
Both known ways of connecting the piston by using the effect of a great tightening moment show drawbacks as it will be described in the following. The great moments necessary during assembly and detachment can only be applied by special tools. The piston or an additional securing element has to include respective engagement surfaces for the special tool. Additional components for realizing an additional element securing against unintentional rotation are necessary, and they result in the structural length in the region of the piston being increased. The front surfaces of the piston and of the piston rod substantially extend in a radial direction, and they contact one another. The front surfaces have to be hardened in a rather complicated way.
Such known piston rod connections are complicated concerning manufacture, and they are rather difficult concerning repairing and servicing. To be capable of arranging the required static seal between the piston and the piston rod, the piston and the piston rod are designed such that they overlap in an axial direction to an extent necessary for arranging the static seal. In this way, the surface of the piston rod which has been produced which great care is also used as a counter surface for the static seal. However, the axial length of the piston is increased in a disadvantageous way, and the piston rod is respectively shortened. This has to be compensated in another way.
Another piston and cylinder unit is known from U.S. Pat. No. 3,293,993. The known piston and cylinder unit includes a housing, a piston having an axis and a piston rod having an axis. A static seal is located between the piston and the piston rod. The piston includes a plurality of piston seals also serving as guiding elements, or which may be replaced by guiding elements by a person with skill in the art. The piston with an inner thread being located in an opening is connected to a protrusion of an outer thread being located on the piston rod. The axes of the threads and the axes of the piston and the piston rod coincide. The known piston and cylinder unit does not include a unit for preventing rotation of the piston with respect to the piston rod, and U.S. Pat. No. 3,293,993 is not related to a design being safe and easy to be repaired.
Another piston and cylinder unit serving to simplify repairing of the arrangement of the piston is also known in the art. The piston and the piston rod are connected by screwing an outer thread loosely into an inner thread. It is also necessary to arrange a static seal being effective in the region of the surface of the piston rod. The known piston and cylinder unit includes a unit for preventing rotation of the piston with respect to the piston rod. The connection of the piston and the piston rod is realized without applying a tightening moment. A bolt, a bush and the like is used as the unit for preventing rotation, the bolt, the bush and the like protruding into an axial bore being arranged such that it protrudes through the threaded portion between the piston rod and the piston. The bush, the bolt and the like also has to be secured against axial movement. Usually, this is achieved by a screw including a securing disc and protruding through the piston in an axial direction. The screw including the securing disc forms a stop against movement of the bush, the bolt and the like. Such an arrangement of the piston requires use of a comparatively great number of additional components serving to prevent rotation. Due to the design and arrangement of the elements, one attains a comparatively long structural length in the region of the connection between the piston and the piston rod. The known arrangement and attachment of the piston is rather complicated and expensive taking the production of the required separate elements into account.